It is known to connect network devices or units together in a stack and to provide a ring or cascade whereby packets received on one of the stacked units can be forwarded from a port on another of the units after traversing the ring or cascade. One example is described in copending application by Brewer et al, Ser. No. 09/207,655 filed 9 Dec. 1998, and incorporated by reference herein.
In that system and in other proposals for a ring connecting stacked units, packets may travel only in a single direction around the ring.
It is customary in network switches, the term including where appropriate devices such as bridges and routers, to establish by examination of the address data in incoming packets and by other procedures to be described a forwarding database which relates a destination address in a packet with related data, including for example the port number associated with that destination address, so that a packet which is received by the switch and includes that address will be forwarded from the correct port, usually after temporary storage within the switch. A forwarding database is normally compiled by relating the source address in a data packet with the port on which the packet was received, so that on reception of a packet which has as its destination address the learnt source address the forwarding database will provide the port number for the forwarding of that packet.
Forwarding databases are well known and employ either media access control addresses, as in ‘bridges’, or protocol addresses as in ‘routers’, or both. Recent examples are described in copending patent application Ser. Nos. 09/286,469 and 09/286,470 for O'Connell, filed 6 Apr. 1999.
Whatever the form of the forwarding database, if a packet is received and examination of that packet does not yield an entry for its destination address in the database, it is necessary to ‘broadcast’ that packet either to all the members of a network or to all the members in at least part of the network, when for example the network is artificially partitioned into ‘virtual’ local area networks. Furthermore, it is frequently necessary or desirable to forward a packet to more than one destination, this form of transmission being a multi-cast packet.
Links between network devices may be either half-duplex, where transmission in one sense only at a time is allowed, or full duplex, wherein simultaneous transmissions in both directions are permitted. Depending on the nature of the transmission medium, duplex working may require physically different lines between the ports connected by a duplex link. In any event, full duplex working inherently provides greater bandwidth and therefore greater information carrying capacity than a half-duplex link.
It is physically possible to connect a multiplicity of devices in a ring wherein all the links of the ring are full duplex, each device in the ring having two ports capable of duplex working, each of the ports being connected to a port on a respective other unit in the ring. However, such a connection has not hitherto been compatible with full duplex working, because, for example, ‘broadcast’ packets would continuously travel around the ring, producing a situation known as ‘lock up’ and preventing the flow of other traffic between the devices. It would however be desirable to be able to form an Ethernet ring with full duplex working around the ring since, as noted above, the full duplex working would increase the bandwidth and therefore the information-carrying capacity of the ring. Furthermore, such an arrangement, if feasible, would have the advantage that if one link failed in the loop there would remain a further connection between the devices and therefore data could still pass between the devices. Moreover, if one device failed, there would still remain a connection between all the other devices in the ring even though full duplex working around the ring could not be maintained. If existing configurations were used to provide the same functionality, then each device would require two connections, trunked together, to each of the devices adjacent to it. However, there would remain two end devices, conventionally termed ‘top’ and ‘bottom’ devices, which could not be connected together and if an intermediate device failed then the top and bottom devices would not possess connectivity with all the other devices.
The present invention therefore has the object of enabling a closed ring connection of Ethernet network devices (hereinafter called ‘units’) providing full duplex working around the ring that is to say on all the links between the units which are connected in the ring.